Diplodia rot of citrus

Early symptoms include bark discoloration, followed by drying and longitudinal cracking of the bark on trunks and branches. These areas often develop into sunken, necrotic lesions that can girdle the affected limb.

The disease is commonly referred to as stem-end rot when it affects fruits. The area around the fruit stalk turns brown or black, becoming leathery, and eventually, the decay progresses through the entire fruit interior.

Leaves on infected branches show signs of chlorosis and may wilt or drop prematurely. The canopy thinning is a clear visual indicator that the tree's vascular system has been compromised by the fungus.

Gummosis, or the oozing of sap from cracks in the bark, is a common defensive response. However, it rarely stops the pathogen from expanding within the woody tissues of the citrus tree.

Under humid conditions, small, black, pinhead-sized structures known as pycnidia erupt through the bark or fruit skin. These signify that the fungus has completed its cycle and is ready to release new spores.

The causative agent of this disease is the fungus Diplodia africana (formerly known as Diplodia natalensis or the anamorph of Botryosphaeria dothidea). It is a persistent pathogen capable of surviving in dead wood, bark, and plant debris for long periods.

As a facultative parasite, the fungus primarily targets stressed or weakened trees. Its spores are disseminated by wind, rainwater splash, and human activity during orchard maintenance, entering the host through wounds or natural openings.

The fungal lifecycle involves the production of pycnidia within infected tissues. These structures release conidia, which act as primary inoculum for secondary infections under favorable environmental conditions.

The mycelium of Diplodia africana invades the xylem, disrupting water and nutrient transport. This systemic movement makes the infection particularly damaging as it slowly kills branches from the inside out.

Laboratory diagnosis often involves isolating the pathogen on agar plates or examining samples for the presence of dark, thick-walled conidia. Precise identification is necessary to distinguish it from other related fungi affecting citrus.

High humidity and prolonged periods of rainfall are the most critical factors for the development of Diplodia rot. Moisture allows the spores to germinate and infect the plant tissues effectively.

Optimal temperatures for the fungus range between +20°C and +30°C. Within this range, the mycelial growth is at its peak, leading to rapid colonization of both fruit and wood tissues.

Tree stress, induced by drought, improper irrigation, or nutrient deficiency, significantly increases susceptibility. A tree with a compromised immune system cannot mount an effective defense against the fungus.

Physical injuries caused by pruning, heavy wind, or insect feeding provide entry points for the pathogen. Once inside, the fungus can colonize the vascular tissue much more easily than on healthy, intact surfaces.

Poor orchard sanitation, such as leaving pruning debris in the aisles, maintains a high inoculum level in the environment. This increases the likelihood of outbreaks after every rainy event.

Diplodia rot is responsible for significant branch dieback and overall tree decline. In severe cases, the entire tree can be lost, leading to long-term reductions in orchard productivity and yield.

The disease causes substantial post-harvest losses. Infected fruit may appear healthy at harvest but rot rapidly during transit or storage, often contaminating entire shipping containers if not managed properly.

Systemic damage to the vascular system weakens the tree's vigor, making it more prone to secondary pests and environmental extremes. This leads to a vicious cycle of increasing decline over several growing seasons.

Because the fungus is difficult to eradicate once it has established itself deep in the woody parts of the tree, control efforts are often costly and labor-intensive, impacting the farm's financial viability.

Continuous monitoring is required because of the latent nature of the infection. Without early detection, the pathogen can spread quietly throughout an orchard before visual symptoms become obvious.

Strict orchard sanitation is the primary control strategy. All dead or dying wood should be pruned out, removed from the orchard, and destroyed to eliminate the source of spores.

Disinfecting pruning tools between trees using a 1% copper-based solution or ethanol is essential to prevent the mechanical transmission of the pathogen during maintenance work.

Applying copper-based fungicides after pruning helps protect open wounds and prevents spore germination. Strategic applications during rainy seasons can significantly lower infection rates.

Maintaining optimal soil fertility and irrigation practices is crucial to keep trees in high vigor. A healthy, well-nourished tree is significantly more resistant to fungal infections than a stressed one.

Effective pest management is necessary to prevent bark-boring insects. By keeping the bark surface intact and healthy, growers can limit the number of available entry points for Diplodia africana.